The present invention relates to methods of applying signal voltages to an electrostatic recording multiple electrode assembly. More particularly, the invention relates to a method of applying signal voltages to an electrostatic recording multiple electrode assembly in such a manner as to produce a high quality image which is recorded with a positive discharge and which is free from missing dots.
In general, in an electrostatic recording device, a high voltage video signal is applied to a multiple stylus electrode head to cause the formation of a latent image on an electrostatic recording sheet or a recording medium through corona discharge. The latent image thus formed is rendered visible by the application of a developer. Such a device, which has a simple construction, has been extensively employed in facsimile devices and other types of printers. For this type of device, a multi-stylus electrode assembly of the same-plane control type or back-plane control type may be employed. However, for simplification of description, the invention will be described with reference only to a case where a same-plane control type multi-stylus electrode assembly is used.
FIG. 1 is a plan view of a conventional multiple stylus electrode assembly of the same-plane control type. In FIG. 1, reference character F designates a main electrode assembly and G an assembly of control electrodes S.sub.1, S.sub.2, . . . and S.sub.n arranged on both sides of the main electrode assembly F. Lead wires 1c, 2c, . . . and nc are connected to the control electrodes S.sub.1 through S.sub.n, respectively, as shown in FIG. 2. The lead wires 1c through nc are connected to a high voltage switching circuit which sequentially applies to the lead wire signals from a ring counter or the like. As the switching circuit is operated, the control electrodes S.sub.1 through S.sub.n are sequentially activated (turned on); that is, high voltage pulse signals 1c', 2c' . . . and nc' of equal pulse width, as shown in FIG. 3, are applied to the control electrodes.
The main electrode assembly F includes electrodes 1A, 2A, . . . and nA in an "A" block arranged confronting the conrol electrodes S.sub.1, S.sub.3, . . . and S.sub.2n-1, respectively, and electrodes 1B, 2B, . . . and nB in a "B" block arranged confronting the control electrodes S.sub.2, S.sub.4, . . . and S.sub.2n, respectively. By sequentially activating the high voltage switching circuits connected to the main electrodes in the "A" block and to the main electrodes in the "B" block, high voltages are applied to the main electrodes of the "A" and "B" blocks with timing as shown in FIG. 3.
In the case where a same-plane control type multi-stylus electrode assembly thus constructed is used to form a latent printing image on a recording medium, as shown in FIG. 4, the polarity of a voltage E.sub.s applied to the control electrodes is made opposite to that of a voltage E.sub.c applied to the main electrodes confronting the control electrodes as shown in FIG. 4, so that the potential difference between the control electrodes and the main electrodes is increased. However, with this technique, the printed image configuration tends to vary according to the polarity of the voltage applied to the main electrode, as will be described in further detail below.
As used herein, an operation of applying a positive potential to the control electrodes while a negative potential is applied to the main electrodes will be referred to as "negative discharge", and an operation of applying a negative potential to the control electrodes while a positive potential is applied to the main electrode will be referred to as "positive discharge". With these definitions, the relationships between applied voltages and recorded dot diameters for positive discharge and for negative discharge are as indicated in the graph of FIG. 5 where stylus diameter is plotted as a parameter.
As is apparent from FIG. 5, it is desirable for achieving a high resolution that the difference between the stylus diameter and the printed dot diameter be minimized. For a given stylus diameter, positive discharge is preferable because the dot diameter for positive discharge is smaller.
FIG. 6 indicates the probability of occurrence of a missing dot at a particular point with the number of dots adjacent to that point, which are recorded simultaneously with the dot at that point, as a parameter. The probability K of a dot being missing (that is, the failure to generate a dot which should actually be printed) is in inverse proportion to the number of adjacent dots. When the number of adjacent dots is one, the probability K can be as large as 40%.
What can be inferred from FIGS. 5 and 6 is summarized as follows: Positive discharge is superior to negative discharge in that the former provides a higher resolution. However, positive discharge still suffers from difficulties in that, in the case where the number of printed dots in an area is small, specifically where the printing of dots is discontinued in the main scanning direction and, for instance, only one dot is to be printed in a particular area, the probability of dots being missing is high, and accordingly it is rather difficult to obtain an accurately reproduced image.
Thus, positive discharge is preferable for a high resolution recording operation. However, it is required that an image printed using positive discharge be high in quality, specifically, free from missing dots. Missing dots can be prevented by increasing the applied voltage. However, it is impossible to increase the voltage applied to the main electrodes because corona discharge will occur between the electrodes. If the voltage applied to the control electrodes is increased, recording tends to be effected with a "white" signal; in other words, a so-called "ghost" image is created which degrades the overall image. Especially for an image for which high resolution is required such as a character composed of a single line, it is essential that each dot be printed. In such an image, if dots are missing, the resultant image is extremely low in quality.
Accordingly, an object of the invention is to provide a method of applying signal voltages to an electrostatic recording multi-stylus electrode assembly, in which the above-described difficulties have been eliminated so that a high quality image is recorded which has no missing dots.